In recent years due to the cost increase associated with producing electricity from fossil fuels, renewable energy technology has gained interest. In particular, among the plurality of renewable energy technologies, Concentrator Photovoltaic (CPV) technology has been the subject of much research. The advantage of CPV over the non-concentrator photovoltaic technology results from the fact that CPV can produce the same amount of electricity as a much larger non-concentrator photovoltaic cell, by focusing the sunlight via a lens on a smaller active semiconductor area. Thanks to this approach, it is possible to reduce the costs associated with the manufacturing of the photovoltaic cell since the amount of materials used is reduced.
However, by concentrating the sunlight in such a manner, CPV systems have a tendency to increase their temperature during operation. This negatively affects the efficiency of the photovoltaic conversion. Accordingly, it is often necessary to position CPV cells on top of structures capable of removing excessive heat from the cells such as passive or active heat sinks.
Additionally, it is customary to place several CPV cells in series. In this case, it is further customary to connect a bypass diode to each of the cells so as to avoid reverse voltages, when only some of the series-connected cells are illuminated by sunlight, which can result in damage to the cell.
Those two requirements, the placement of the CPV cells on a heat sink and the connection of a bypass diode to each CPV cell, can be solved by the structure represented in FIG. 4A.
In particular, FIG. 4A illustrates a cross-sectional view of a solar cell assembly 6000, while FIG. 4B illustrates a top view of the same solar cell assembly. More specifically, the solar cell assembly 6000 comprises a heat sink 6100 made of thermally and electrically conductive material on top of which a photovoltaic cell 6420 is mounted. The photovoltaic cell 6420 comprises a plurality of semiconductor layers, schematically represented as stacked layers 6421-6424, for instance, a plurality of p-doped and n-doped layers. In particular, the photovoltaic cell 6420 can be a III-V concentrator photovoltaic cell. The solar cell assembly 6000 further comprises diode 6220.
Both diode 6220 and photovoltaic cell 6420 are positioned on, from top to bottom, a substrate 6230, 6430, a metal contact 6240, 6440, and an electrically and thermally conductive glue, solder paste or adhesive layer 6250, 6450, respectively. Further, both diode 6220 and photovoltaic cell 6420 comprise a front contact 6210, 6410, respectively, on their upper surface. An electrical back contact of both diode 6220 and photovoltaic cell 6420 are made through the substrate 6230, 6430, the metal contact 6240, 6440, the glue layer 6250, 6450 to the electrically conductive heat sink 6100.
In each diode-photovoltaic cell couple, the diode 6220 and photovoltaic cell 6420 are connected in an antiparallel manner. One connection is achieved via a wirebond connection 6300 between front contacts 6210 and 6410. The remaining connection is achieved through the heat sink 6100. The heat sink 6100 is electrically conductive in order to be able to report the back contact to the electrical contact pad 6110. As an example, a standard Ge/GaAs/InGaP multi-junction solar cell utilizes an electrically conductive Ge substrate on which the other junctions are grown by epitaxy. Additionally, the diode 6220 and photovoltaic cell 6420 are connected to neighboring diode-photovoltaic cell couples by wirebond connections 6310 and 6320. In particular, point A of a first couple is connected to point B of a second couple and so on, so as to realize a series connection of a plurality of diode-photovoltaic cell couples.
This arrangement requires all layers 6230-6250 and 6430-6450 to be electrically conductive. Additionally, all those layers must be thermally conductive as well, since heat has to be dissipated, mostly from the photovoltaic cell 6420, via the different elements, into the heat sink 6100. Still further, the solar cell assembly realized in this manner is costly since the photovoltaic cell 6420 and the diode 6220 are provided as separate elements, and many individual manufacturing steps are required, for instance, pick-and-place processes for each cell and each diode.
Patent document US 2010/0243038 discloses (cf. FIG. 1) a solar cell assembly 10 in which a substrate 12 is used as a carrier for a multi junction solar cell 20. The top of the substrate 12 comprises a diode 18 and the solar cell 20 is attached to the substrate 12 via a conductive bonding material 34, such as silicone, epoxy, solder or braze (cf. paragraph [0030]).
However, this does not solve the above-mentioned problems, as the bonding material 34 presents, as layers 6230-6250 and 6430-6450, both an electrical and thermal resistance. Further, the positioning of the diode 18 along the entire top surface of substrate 12 renders the placement of contacts 42, 40 and 46 difficult for manufacturing. In particular, contact 40 exposed from substrate 12 cannot be realized with standard semiconductor technology. Additionally, connection to contact 42 on the back side of substrate 12 is difficult. Even further, if the substrate 12 is mounted on a heat sink, contact 42 is electrically connected to the heat sink, which may be undesirable in some cases.